Wearable devices utilize sensors to monitor analytes in human biofluids to reflect physiological performance in sports or an aberrant component imbalance in diseases such as diabetes mellitus and cystic fibrosis. Such devices have the potential to revolutionize healthcare and disease management as a surrogate strategy for measuring circulating analyte concentrations in the blood, and especially in a dynamic, noninvasive manner.
Recent advances have led to multiple sensors for mechanoelectrical transduction using nanoparticles, carbon nanotubes, liquid metals, and ionic liquids . The key developments to date have mainly focused on skin interstitial fluid (ISF), tears, saliva, and sweat.
Biomarkers in Tears
Human body fluids and secretions contain molecules known as biomarkers that contain a wealth of information about the body’s health and the presence of disease. Among secretions such as tears, sweat, and saliva, tears are considered the best source of biomarkers, with concentrations similar to those found in the blood. Tears are also sterile, readily available, and less susceptible to the damaging effects of temperature change, evaporation, and the rate of secretion.
Useful and measurable biomarkers found in tears include sodium ions, which are useful indicators of dry-eye disease, and glucose molecules, an early diagnostic tool for diabetes. Also, measuring the pH of tears can be used to check for cell viability, drug effectiveness, and signs of disease.
Researchers at the Terasaki Institute in Los Angeles have developed a contact lens prototype that can help collect tear samples for diagnostic purposes and also improve tear flow to prevent possible dry eye disease. It contains a hydrogel lens, micro channels through which tears can pass, and test chambers where electrochemical and colorimetric tests can evaluate sodium levels and pH, respectively.
Tear samples can be difficult to collect regularly, so one way to evaluate tears in real time involves biosensing contact lenses. The newly developed lens contains tiny micro channels that direct tears to the sampling and test chambers; where an electrochemical test evaluates the samples for sodium levels and a simple color change reveals the tear pH.
New Hydrogel Contact Lenses (Microengineered Poly -HEMA- Hydrogels)
Until now, it has been difficult to fabricate microchannels in hydrogel lenses because the manufacturing processes involved are not suitable for such fine properties. The Terasaki researchers achieved this by optimizing the gel to have an elastic and smooth surface, and then used a 3D-printed mold to introduce the microchannels.
Once the successful prototype was completed, it was extensively tested for its performance in channeling and collecting fluids. Flow rates of artificial tears in the channels were measured at different levels of hydration, with zero flow measured at complete dehydration and full spontaneous flow observed at full hydration.
“In addition to our successful fabrication of microchannels in commercial contact lens hydrogels, we also found that eye-blinking pressure may facilitate tear exchange in the lens through these microchannels,” said Shiming Zhang, Ph.D., from the Terasaki Institute’s research team. “This is an exciting finding because it opens the possibility for the lenses to be a means of preventing dry eye disease, a condition commonly found in contact lens wearers. We aim to develop a patented contact lens that actively treats this condition by enhancing tear flow in the eye.”
Biomarkers in Tears to Monitor Patients Health
The team next prototyped sensors to collect, test for and measure pH levels of artificial tears flowing through the microchannels. Sodium levels were also tested, and the results showed an acceptable and predicted range of sodium detection for diagnostic purposes.
“The production of the successful prototype described here, and the continuing efforts to perfect its capabilities mark a significant advance in contact lens biosensing,” said Ali Khademhosseini, Ph.D., director, and CEO of the Terasaki Institute. “Such innovative work fits in well with our institute’s mission to create solutions that restore or enhance the health of individuals.“